Rotten has some great tuts Here. Especially for wrinkles.
If you post a pic of the t-shirt you are working on you may get better responses on what to do to make it better. Just a snap shot of the t-shirt on your avatar would be great._________________

If you are using photoshop go to your fill tool, select pattern, then try one of your rock textures, like slate or something. Then set it to overlay or a hard light. Make a new layer over this in a medium gray. set to overlay, and use your dodge and burn tools with medium soft brushes on about 10 to 20 strength to accent your shadows and highlighted areas. That will vary according to if your shirt is for male or female and what mesh you are using. For girls clothing I always like to keep the mcg guide under my wire frames to give me an idea of how I want to highlight._________________You want the truth? You can't handle the truth.

Wow Verses, great answer. I was just going to say that i simply use a flat color background, with a pattern overlay (I usually use herringbone) to give it some cloth look. Experiment with this until you get the look that you want. Then I will create front and back images separately until the shirt is complete.